1. Field of the Invention
The present invention relates to a method and apparatus for detecting failure of a differential transformer used with an electric micrometer, etc.
This invention also relates to a method and apparatus for signal processing of a differential transformer for obtaining a measurement signal from the differential transformer. Particularly, the present invention relates to a method and apparatus for signal processing of a differential transformer suitably when small displacement measuring instruments such as electric micrometers are incorporated inline for use.
2. Description of the Related Art
As a displacement gauge for measuring displacement of the object to be measured, an electric micrometer is used for inline multipoint measurement as illustrated in FIG. 11.
In FIG. 11, numeral 10 denotes an electric micrometer containing a differential transformer, numeral 12 denotes a measurement unit, numeral 12A denotes offset and gain adjustment controls. Further, numeral 14 denotes a unit case in a built-in manner, numeral 14A denotes a selector, numeral 14B denotes a check meter, and numeral 16 denotes a personal computer (PC).
As shown in detail in FIG. 12, in the electric micrometer 10, displacement of a spindle 22 forming a part of a probe 20 is transmitted to a core 26. Sensor coils 28 and 30 are placed symmetrically with respect to the mechanical center position of the core 26 (called neutral point) and connected in series so as to form a differential transformer. The electric micrometer 10 uses change in the impedance of the sensor coils 28 and 30 depending on the position of the core 26 to detect displacement of the spindle 22. That is, the electric micrometer 10 detects displacement of the spindle 22 based on change depending on the position of the core 26 in the difference between voltages E1 and E2 (E1-E2) as shown in FIG. 13 occurring across each of the sensor coils 28 and 30 when a voltage is applied to the sensor coils 28 and 30 from an external oscillator 32.
Specifically, as shown in FIG. 14, a drive signal shaped like a sine wave generated in the oscillator 32 is applied to the sensor coils 28 and 30 through a transformer 40, for example. An amplifier 46 amplifies output at the neutral point of the sensor coils 28 and 30 and output of a variable resistor (called control) 42 for zero adjustment (offset correction). Output of the amplifier 46 is input through a span adjustment (gain correction) control 48 and a capacitor 50 for offset removal to a synchronous rectifier 52, which then half-wave or full-wave rectifiers the input in synchronization with the drive signal output by the oscillator 32. Then, output of the synchronous rectifier 52 is smoothed through a filter 54 and the analog signal output through the filter 54 is converted into a digital signal by an analog-digital (A/D) converter 56. Then, the digital signal is displayed in digital form on a display 58 implemented as a liquid crystal display (LCD), for example. In FIG. 14, numeral 44 denotes a resistor. However, in the electric micrometer using such a differential transformer, the sensor signal becomes zero at the mechanical neutral point of the probe 20 (core 26) and thus it is difficult to discriminate between a failure caused by a broken wire, etc., and the normal signal from the neutral point; this is a problem.
Further, to conduct high-accuracy measurement, the frequency stability and oscillation stability of the oscillator 32 need to be high, the amplification stability of the amplifier 46 needs to be high, and offset needs to be low.
However, the parts accuracy of the coil and capacitor used with the oscillator 32 for generating the sine wave to drive the sensor coils 20 and 30 are insufficient. Further, it becomes necessary to adjust the oscillation frequency and amplitude with trimmer control at the assembling time and the calibration time, and the frequency and voltage easily change due to temperature change. On the other hand, high-accuracy parts are expensive and moreover involve a problem of being still hard to raise stability.
Offset correction to set the origin and adjust the zero point as shown in FIG. 15 is made with the control 42. Gain correction to adjust the measurement span using a master work, etc., as shown in FIG. 16 is made with the control 48. However, the two controls 42 and 48 affect each other and thus adjustments are hard to make. Since the controls are used, the effects of temperature change and secular change are easily received. Further, since the control attachment space is required, there is a problem of being hard to miniaturize, etc.
Particularly, to conduct multipoint measurements with the electric micrometers incorporated inline, it is substantially almost impossible to make manual offset correction and manual gain correction for each probe.
Further, in the synchronous detection system using the synchronous rectifier 52, to deal with the various types of probes and lengths of signal cables, a phase shift occurs in an output signal to the probe and an input signal from the probe. FIG. 17A shows phase change depending on the probe type and FIG. 17B shows phase change caused by the cable length difference. Therefore, due to the phase shift occurring in the output signal to the probe and the input signal from the probe, synchronous rectification cannot well be conducted and the waveform may get out of shape as shown in the upper stage of FIG. 17C.
When the A/D converter 56 converts an analog signal into a digital signal, unless a filter having a large time constant is inserted, it is hard to suppress display flicker as shown in FIGS. 18 and 19 and improvement in the response speed is inhibited. This can lead to a fatal problem to incorporate the electric micrometers inline for use for automatic control in addition to simple display.
The invention is intended for solving the above-described problems in the related art.
It is a first object of the invention to provide a method and apparatus for failure of a differential transformer which can discriminate between the break state in an input signal line or a drive signal line and the normal state in the differential transformer.
It is a second object of the invention to provide a method and apparatus for signal processing of a differential transformer which can be incorporated inline for use for automatic control like a linear encoder by reducing the effects of temperature change and secular change, eliminating the need for adjustment at the assembling time, and facilitating calibration.
It is a third object of the invention to provide a method and apparatus for signal processing of a differential transformer which can deal with various types of probes, cable extension, etc.
It is a fourth object of the invention to provide a method and apparatus for signal processing of a differential transformer which can improve the response speed and suppress display flicker, etc., even with a filter having a small time constant.
In order to achieve the first object of the invention, there is provided a method for detecting failure of a differential transformer comprising: applying an offset signal for failure detection, that cannot be removed if an input signal line of the differential transformer is broken, from an offset application circuit to an output signal of the differential transformer, the offset application circuit having an output impedance higher than an impedance of the differential transformer.
Further, in order to achieve the first object of the invention, there is provided an apparatus for detecting failure of a differential transformer comprising: an offset application circuit for applying an offset signal for failure detection, that cannot be removed if an input signal line of the differential transformer is broken, to an output signal of the differential transformer, the offset application circuit having an output impedance set higher than an impedance of the differential transformer; an offset removal circuit for removing an offset from the output signal to which the offset signal is applied; a rectification circuit for rectifying an output signal of the offset removal circuit; and a level determination circuit for detecting the input signal line or a drive signal line of the differential transformer being broken based on an output level of the rectification circuit.
Preferably, the above-mentioned failure detection apparatus of the differential transformer further comprises: a signal processing portion for outputting an alarm to the exterior and holding an immediately preceding measurement data of the differential transformer if the signal line or the drive signal line of the differential transformer being broken is detected.
In order to achieve the second object of the invention, there is provided with an apparatus for signal processing of a differential transformer for obtaining a measurement signal from the differential transformer, comprising: a drive signal generation section for generating a drive signal to be applied to the differential transformer; and a signal processing section for obtaining the measurement signal from an output signal from the differential transformer, wherein at least a part of the drive signal generation section and the signal processing section is digitalized.
In the above-mentioned signal processing apparatus of the differential transformer, it is preferable that the drive signal generation section includes an oscillator for generating a clock signal, and a memory for storing a waveform data called in synchronization with the clock signal of the crystal oscillator.
Further, in the above-mentioned signal processing apparatus of the differential transformer, it is preferable that the signal processing section includes an output correction portion for making an offset correction and a gain correction to the output signal of the differential transformer in digital form.
Preferably, the offset correction and the gain correction are made externally.
In order to achieve the third object of the invention, in the above-mentioned signal processing apparatus of the differential transformer, the signal processing section includes a phase adjustment portion for digitally compensating for a phase shift in the output signal of the differential transformer
In order to achieve the fourth object of the invention, in the above-mentioned signal processing apparatus of the differential transformer, the signal processing section includes a digital filter for providing hysteresis to the output signal of the differential transformer.
Preferably, the signal processing apparatus of the differential transformer further comprises a data communication function between electric units.